Downhole signal source

ABSTRACT

A signaling apparatus comprises a magnet and a shield moveable relative to the magnet. The shield is moveable relative to the magnet between a first position in which the magnet is relatively exposed and a second position in which the magnet is relatively shielded. The apparatus can include a synchronization signal source, a downhole sensor signal source, and/or means for modulating the magnetic field in response to the signal from any source. A method of using the signaling apparatus to locate a bottomhole assembly includes moving the shield so as to modulate the magnetic field created by the magnet, sensing the modulation of the magnetic field, and determining the location of the bottomhole assembly using the information collected. The BHA can be located using phase shift or amplitude measurements. Receivers detecting the modulated magnetic field can be at or below the earth&#39;s surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.10/856,439, filed May 28, 2004, entitled “Downhole Signal Source” whichis incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

GENERAL FIELD OF THE INVENTION

The present invention relates generally to methods and apparatus forsignaling from one location to another using low frequency magneticfields. The invention can be used to send a signal from a location neara drill bit in a well drilling operation to a receiver at the earth'ssurface, or to a receiver at a different location in the drillstring inthe same well, or to a receiver in another well. These and otherfeatures of the invention are described in detail below.

BACKGROUND OF THE INVENTION

In common practice, when it is desired to produce hydrocarbons from asubsurface formation, a well is drilled from the surface until itintersects the desired formation. As shown in FIG. 1, a typical drillingoperation entails a surface operating system 50, a work string 100 thatmay comprise coiled tubing or assembled lengths of conventional drillpipe, and a bottom hole assembly (BHA) 200. Surface system 50 typicallyincludes a drilling rig 10 at the surface 12 of a well, supporting drillstring 100. BHA 200 is attached to the lowermost end of work string 100.Operating system 50 is positioned at the surface adjacent to well 12 andgenerally includes a well head disposed atop of a well bore 18 thatextends downwardly into the earthen formation 20. Borehole 18 extendsfrom surface 16 to borehole bottom 30 and may include casing 22 in itsupper zones.

The productivity of formations can vary greatly, both vertically andhorizontally. For example, in FIG. 1, formation 21 may be a producingformation (stratum), while formation 20 above it may be a non-producingformation. The target formation(s) have typically been mapped usingvarious techniques prior to commencement of drilling operations and anobjective of the drilling operation is to guide the drill bit so that itremains in the target formation. Thus, in many wells, the lower portionof the borehole deviates from the vertical and may even attain asubstantially horizontal direction. In these circumstances, it isdesirable to drill the well such that borehole 18 stays within theproducing formation 21.

Similarly, it is sometimes desired to guide the drilling of a well suchthat it parallels another well. This is the case in steam-assistedgravity drainage (SAGD) drilling, in which steam injected through one ofa pair of parallel wells warms the formation in the vicinity of thewells, lowering the viscosity of the formation fluids and allowing themto drain into the second well. The second well thus functions as aproduction well and typically is drilled such that it lies below theinjection well.

As a result of this deviated, directional, or horizontal drilling, thedrill bit may traverse a sizable lateral distance between the wellheadand the borehole bottom. For this reason, and because the degree ofcurvature of the borehole is often not known precisely, it also becomesdifficult to know the true vertical depth of the borehole bottom. Hence,it is preferred to track the position of the bit as precisely aspossible in order to increase the likelihood of successfully penetratingthe target formation.

It is particularly desirable to accurately locate the position of thebottom hole assembly (BHA) during drilling so that corrections can bemade while drilling is ongoing. Determining the precise location of thedrill bit as it progresses through the formation and communication ofthat information from the downhole location to the surface are twosignificant problems that have not heretofore been adequately addressed.Both objectives are made more difficult by the drilling operationitself, which involves at least rapid fluid flow, moving parts, andvibrations.

Various methods are traditionally combined to achieve these goals.Gyroscopes and various types of sensors have been used to track bitmovement and/or bit position. Electromagnetic (EM) telemetry is onetechnique used for transmitting information, either to the surface or toanother location uphole. Other transmission techniques involve mudpulses or acoustic signaling using the drillstring as the signalcarrier. Current techniques are not very accurate or rapid, however, andcan result in erroneous calculations of the position of the BHA. Hence,it is desirable to provide a technique for determining the position of abit in a subterranean formation that eliminates or at leastsubstantially reduce the problems, limitations and disadvantagescommonly associated with the known bit-tracking techniques.

SUMMARY OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention provides methods and apparatus for signaling fromone location to another using low frequency magnetic fields. Theinvention has many applications and can be used, for example, to locatethe position of the bottom hole assembly during drilling. The inventioncan be used to send a signal from a location near a drill bit in a welldrilling operation to a receiver at the earth's surface, or to areceiver at a different location in the drillstring in the same well, orto a receiver in another well. The invention can also be used forgenerating a signal at the earth's surface that can be detected at adownhole location, or as a telemetry transmitter for low frequencycommunications.

In some embodiments, the apparatus of the present invention isparticularly useful as a tool for sending a signal from the bit locationthat can be detected at the surface and used to determine the locationof the bit. The present invention avoids the deficiencies of priordevices and offers an alternative way to determine the position of theBHA. In preferred embodiments, the invention includes placing asignaling apparatus at the bit and tracking its position during theentire drilling process. For this method to work, the signal source mustbe strong and stable enough even for deep and extended-reach wells.

In certain embodiments, a synchronization signal and using saidsynchronization signal is provided and used to control modulation of themagnetic field created by the magnet. Controlling the modulation of themagnetic field may include doubling the frequency of, taking theabsolute value of, or squaring the synchronization signal. The modulatedmagnetic field can be sensed by receivers that may detect a phase shiftbetween said synchronization signal and said modulated magnetic fieldand or amplitude variations in said modulated magnetic field. There maybe a plurality of receivers spaced apart from said bottomhole assembly,and the receivers may be located at or below the earth's surface.

In alternative embodiments, the invention can also be used to generate asignal at the earth's surface that can be detected at a downholelocation.

In some embodiments of the present invention, the signal source may be arare earth permanent magnet used in conjunction with a shield made ofhigh permeability soft magnetic alloy. By precisely controlling themotion of the shield, the permanent magnet can be made to function as aprecise oscillating signal source that can be tracked by magnetometersat the surface for accurate position monitoring of the BHA. Inalternative embodiments, the frequency and/or phase etc. of the motionof the shield can be modulated in response to data acquired by downholeinstruments using well-known digital encoding schemes, transforming thesignal source into a transmitter that can communicate LWD data tosurface receivers.

In certain embodiments, the present invention comprises a magnet and ashield moveable relative to said magnet between a first position inwhich said magnet is relatively exposed and a second position in whichsaid magnet is relatively shielded. The magnet can be an electromagnet.The present system may further comprise means for providing asynchronization signal and means for controlling movement of the shieldin response to the synchronization signal so as to modulate the magneticfield created by the magnet. The means for controlling the shieldmovement may include means for doubling the frequency of, taking theabsolute value of, and/or squaring the synchronization signal. Theapparatus may further include a downhole sensor generating a signal andmeans for modulating the magnetic field in response to the signal fromthe downhole sensor.

Thus, the embodiments of the invention summarized above comprise acombination of features and advantages that enable them to overcomevarious problems of prior devices systems and methods. The variouscharacteristics described above, as well as other features, will bereadily apparent to those skilled in the art upon reading the followingdetailed description of the preferred embodiments of the invention, andby referring to the accompanying drawings.

It should be appreciated that the present invention is described in thecontext of a well environment for explanatory purposes, and that thepresent invention is not limited to the particular borehole thusdescribed, it being appreciated that the present invention may be usedin a variety of well bores.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the preferred embodiments of thepresent invention, reference will now be made to the accompanyingdrawings, wherein:

FIG. 1 is a schematic elevation view, partly in cross section, of adrillstring including a bottom hole assembly (BHA) in a subterraneanwell;

FIG. 2 is a simplified perspective view of a signal source in accordancewith a preferred embodiment of the invention;

FIG. 3 is a cross sectional view of the signal source of FIG. 2incorporated into a downhole tool;

FIGS. 4 and 5 are end views of a signal source in accordance with afirst alternative embodiment, in closed and open positions,respectively.

FIG. 6 is a simplified view of a slotted sleeve that can be used incertain embodiments of the present invention;

FIG. 7 is a plot illustrating the dependence of magnetization ontemperature, where Ms is the saturation magnetization;

FIG. 8 is a schematic diagram illustrating an embodiment of a systemincorporating a signal source in accordance with the present invention;and

FIGS. 9A-D are plots illustrating a transmitted signal (A), the samesignal after squaring (B), the squared signal after filtering (C), and acomparison of all three modes through one cycle of the original signal(D).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . ”. Also,reference to “up” or “down” are made for purposes of ease of descriptionwith “up” meaning towards the surface of the wellbore and “down” meaningtowards the bottom of the wellbore. In addition, in the discussion andclaims that follow, it is sometimes stated that certain components orelements are “electrically connected.” By this it is meant that thecomponents are directly or indirectly connected such that an electricalcurrent or signal could be communicated between them.

According to the present invention, the strong magnetic moment of therare earth permanent magnet is used together with the shield made ofhigh permeability soft magnetic alloys. By precisely controlling themotion of the shield, the permanent magnet is transformed into a preciseoscillating signal source that can be tracked by magnetometers at thesurface for accurate position monitoring of the BHA. Alternatively, thespeed/phase etc. of the motion of the shield can be modulated with dataacquired by downhole instruments through well-known digital encodingscheme, thus transform the signal source into a transmitter that cancommunicate LWD data to surface receivers.

Referring now to FIGS. 2 and 3, one preferred embodiment of a signalsource 10 in accordance with the present invention includes a permanentmagnet 12, a magnetic shield 14, and a drive mechanism 16 for shiftingshield 14 relative to magnet 12. Magnetic shield 14 is slidable axiallyinto and out of surrounding engagement with magnet 12, as indicated byarrow 26. Drive mechanism 16 engages one end of shield 14 and providesthe motive force needed to advance and retract the shield. Referring nowparticularly to FIG. 3, signal source 10 is preferably mounted inside acylindrical non-magnetic drill collar 20, along with a drive means 30.The assembly formed in this manner preferably has a central bore 22therethrough such that the drill collar can be included in a drillstring.

In the embodiment shown in FIGS. 2 and 3, magnet 12 is generallycylindrical and shield 14 likewise comprises a cylindrical shell. Shield14 preferably includes an end cap 17 and a cylindrical inner surface 15having a diameter only slightly larger than the outside diameter ofmagnet 12. Shield 14 is preferably moveable between first and secondpositions in which magnet 12 is, respectively, exposed and shielded.

In FIG. 3, shield 14 is shown in an intermediate position, with magnet12 partially exposed and partially shielded. The length of arrow 26illustrates an approximate range of movement for shield 14. As shield 14moves along the length of magnet 12, the fraction of magnet 12 that isexposed changes. Correspondingly, the magnetic field emanating frommagnet 12 changes as shield 14 attenuates it. When magnet 12 is whollywithin shield 15, the magnetic field emanating from the tool 100 will atits minimum. In certain embodiments, the movement of shield 14 relativeto magnet 12 can be controlled so as to produce a sinusoidal modulationof the magnetic field that extends beyond the tool. Likewise, themovement of shield 14 can be controlled such that the magnetic fieldcycles in a sawtooth manner, or according to any preferred function ormodulation.

In an alternative embodiment of the invention, depicted in FIGS. 4 and5, the shield consists of two or more partial circumferential sections40, 42. Sections 40, 42 are preferably configured such that togetherthey can be closed to form a shield that encloses the circumference ofmagnet 13.

In still another embodiment, shown in FIG. 6, the shield can comprisetwo or more concentric cylindrical shells, each generally having theconfiguration shown at 50 and each having a plurality of longitudinalslots 52 therethrough. The magnet is disposed within the innermostshell. When the concentric shells are positioned such that the slots ineach shell are aligned with the slots in the other shell(s), the magnetis exposed. Similarly, when the shells are positioned such that theslots do not align, the magnet is shielded.

It will be understood that the configurations shown herein are merelyillustrative of the manner in which the magnetic material and the shieldcould be configured. Various other arrangements of the components of thetool will be understood by those skilled in the art.

Magnet

In order to have the highest available magnetic energy, rare-earth basedpermanent magnets such as Nd/Fe/B and Sm/Co are preferred. With amagnetic energy (BxH)_(max) in excess of 200 KJ/m³, Nd/Fe/B magnets arethe strongest permanent magnets available today. Sm/Co magnets typicallyhave a lower magnetic energy, at about 150 KJ/m³.

As is known, permanent magnets are made of ferromagnetic materials. Oneof the characteristics of ferromagnetic materials is the existence of acritical temperature (T_(c)) called Curie temperature. Above thistemperature, ferromagnetic materials lose their magnetization and becomeparamagnetic. The transition is gradual within a temperature range; evenbefore the temperature of the magnet reaches its Curie temperature, themagnet starts to lose its magnetization. This behavior can be describedby the molecular field theory that gives the temperature dependencedepicted in FIG. 7. Hence, if a permanent magnet is to maintain 80% ofits magnetization in the downhole environment, it must operate intemperatures no higher than 0.7×T_(c), where T_(c) is the Curietemperature. For Sm₂Co₁₇, T_(c) is 700-800° C., while it is 300-350° C.for Nd₂Fe₁₄B. Therefore, for deep wells where the bottom holetemperature is high, Sm₂Co₁₇ magnets are preferred.

Shield

In order to modulate the strength of the permanent magnet, shield 14 ispreferably made of a magnetically soft alloy such as Mumetal®(Ni/Fe/Cu/Mo) or Supermalloy, with high magnetic permeability. Varioussuitable magnetically soft metals are known in the art, includingCO-NETIC AA®, which has a high magnetic permeability and provides highattenuation, and NETIC S3-6®, which has a high saturation inductionrating that makes it particularly useful for applications involvingstrong magnetic fields. NETIC S3-6 and CO-NETIC AA are trademarks ofMagnetic Shield Corp., 740 N. Thomas Drive, Bensenville, Ill. 60106. Inembodiments where it is desired to achieve very high attenuation ratiosin a very strong field, it may be preferred to use both alloys. In theseinstances, the NETIC S3-6 alloy is preferably positioned closest to thesource of the field so as to protect the CO-NETIC AA alloy fromsaturation. Alternative metals that are suitable for use in shield 14include but are not limited to Amumetal® and Amunickel® from AmunealManufacturing Corp., 4737 Darrah Street, Philadelphia, Pa. 19124, USA.

Motor

Motive force for moving shield 14 relative to magnet 12 is preferablyprovided by drive means 30, which is housed inside drill collar 20.Drive means 30 is preferably an electric motor, but can be any othersuitable mechanical drive device. It will be understood that, dependingon the type of power source selected, it may be necessary to providegearing and the like in order to allow drive means 30 to cause thedesired movement of shield 14, whether that be rotational,translational, or other.

Use of the Downhole Transmitter

As mentioned above, one preferred use for a transmitter of the typedisclosed herein is as a field source for a downhole absolutepositioning system. The purpose of such a system is to allow a precisedetermination of the position of the bottomhole assembly. This can bedone by using the present signal source to generate an ultralowfrequency signal (0.1 Hz to 0.01 Hz, depending on depth, with greaterdepths requiring lower frequencies) that is extremely stable andprecisely synchronized with a surface clock. The transmitter itself canbe a transmitter of the type herein disclosed or a large electromagnet.A highly stable synchronization signal makes it possible to operate in avery narrow bandwidth, which in turn makes it possible to receive thesignal with a minimum of noise and improves the quality of the resultingtelemetry.

When the present invention is used to assist in location of a bottomholeassembly, for example, it is preferably positioned in the drillstringadjacent to the BHA. The present signaling devices may not be inphysical contact with the BHA, but the greater the distance between theBHA and the signaling apparatus, the less precise will be theinformation relating to location of the BHA. Because precise location ofthe signal source is achieved by a combination of phase shift andamplitude measurements, timing is particularly important in thisembodiment.

In other embodiments, the downhole signal source need not besynchronized to an synchronization signal. This type of system can beused when it is desired to generate a signal at the earth's surface thatcan be detected at a downhole location, or when the system is used as atelemetry transmitter for low frequency communications. In still otherembodiments, an array of three or more surface sensors can be usedlocate the signal source using triangulation techniques, with or withouta synchronization source.

In spite of the frequency stability requirement, it is not necessary tocarry a precise clock (good to about 1 millisecond over 200 hours)downhole. Nonetheless, in some embodiments, a downhole clock may bepreferred. In one embodiment, illustrated in FIG. 8, a precise clock 100is located at the earth's surface. Clock 100 is used to synchronize asystem that includes a downhole signal source in accordance with thepresent invention. In the embodiment shown in FIG. 8, clock 100 iselectrically connected to a surface sine wave transmitter 112, which inturn is electrically connected to a surface antenna 114. Clock 100 canbe an atomic clock, a clock obtained from the GPS system, an overcontrolled system of oscillators, or any other suitable precise clock.

Still referring to FIG. 8, a signal 118 from surface antenna 114 istransmitted through the earth and are received at a downhole receiver120. The received signal from the downhole receiver 120 is preferablypassed through a preamplifier 122 into a digital-to-analog converter andthen through signal processing means that use the received signal tosynchronize the downhole system. In a preferred embodiment, the signalprocessing means comprise a CPU 124 that applies a squaring algorithmand a low pass filter to the received signal. CPU 124 also implementscontrol logic that drives a downhole system clock. The output of the lowpass filter is preferably sent to a digital-to-analog (D/A) converter126. The output of D/A converter 126 is preferably amplified by anamplifier and then used to control drive means 30. In embodiments wherean electromagnet is used, the output of the D/A converter can be used tooperate to the electromagnet, preferably with amplification.

Regardless of the source of the drive signal, the signal source 10ultimately generates a signal 130 that comprises a variable magneticfield. Signal 130 is detected by a sensing device 140, which preferablycomprises an array of at least two receivers 142, 144, 146, 148. Sensingdevice 140 may or may not be located near antenna 114. If a surfacesynchronization source is used, the phase and/or amplitude of thereceived signal 130 can be used to locate the signal source.Timing-induced errors can be mitigated by using a digital phase lockloop circuit or other suitable means. In alternative embodiments, thefrequency and/or the phase of signals 130 can be modulated so as totransmit signals from the borehole bottom to the surface, such as, forexample, signals indicative of measurements made by downhole sensorsand/or MWD equipment.

Clock 100 is preferably used to generate a sine wave at one-half thefrequency of the signal that is to be transmitted by the downholetransmitter (FIG. 9A). In an alternative embodiment, the clock signalcan be induced directly into the drillstring and sensed as an electricfield across an insulating gap in the bottomhole assembly or by anyother current-sensing means. It is well known that if a sinusoidalsignal is squared, that the resulting signal contains only evenharmonics of the fundamental signal. In particular, the Fourier seriesrepresentation of a rectified sine wave is given by Equation (1) and isillustrated in FIG. 9B. $\begin{matrix}{{{\sin( {\omega \cdot t} )}} = {\frac{2}{\pi} - {\frac{2}{\pi} \cdot {\sum\limits_{n = 1}^{\infty}{\frac{2}{( {2 \cdot n} )^{2} - 1} \cdot {\cos( {2 \cdot n \cdot \omega \cdot t} )}}}}}} & (1)\end{matrix}$

Whether the procedure is carried out using analog electronics or digitalelectronics, the concept is the same: take the absolute value of thereceived signal (or square it) and low pass filter it (FIG. 9C). Thefundamental frequency of the resulting signal will be exactly twice thatof the transmitter at the earth's surface. The signal will containhigher order harmonics which can be filtered out downhole, if desired(the higher the order of the harmonic, the more this signal will beattenuated as it propagates through the earth, back to the earth'ssurface). FIG. 8 illustrates one possible way of carrying a preferredprocedure out using mostly digital electronics. It should be appreciatedthat the digital functions could be replaced with analog functions ifdesired, but since the frequencies used are so low, the required signalprocessing is well within the capabilities of present technology.

FIGS. 9A-D illustrate the waveforms, individually and together (9D) thatresult in a preferred signal processing technique that is suitable foruse in the present invention. It will be understood that any othersynchronization signal source or other signal processing techniques canbe used in the present invention and that the signal(s) need not besinusoidal.

Advantages

Compared with active sources using active dipole source energized byalternating current, the new signal source will be stronger, morestable, and more accurate. The present signal source can be used toprecisely locate a BHA while drilling. It can also be used to improvedepth reference in wireline logging operations by reducing errorsrelated to cable stretching due to thermal expansion, sticking/stuckwireline tools, etc. Coupled with digital coding schemes, the presentsignal source can also be employed as a transmitter to send downholetool and or formation data to surface receivers, thus provide anadditional communication channel for LWD.

While certain preferred embodiments have been disclosed and described,it will be understood that various modifications may be made theretowithout departing from the scope of the invention. For example, thetype, size and configuration of the magnet and of the shield can bevaried. Likewise, the mode of movement of the shield relative to themagnet can be altered or varied. To the extent that the claims include asequential recitation of steps, it will be understood that those stepsneed not be completed in order and that it is not necessary to completeone step before commencing another.

1. A downhole tool, comprising: a magnet; and a shield moveable relativeto said magnet.
 2. The tool according to claim 1 wherein said magnetcomprises a permanent magnet and said magnetic shield is moveablerelative to said magnet between a first position in which said magnet isrelatively exposed and a second position in which said magnet isrelatively shielded.
 3. The tool according to claim 1 wherein saidmagnet comprises an electromagnet.
 4. The tool according to claim 1,further including means for providing a synchronization signal and meansfor controlling movement of said shield in response to said signal so asto modulate the magnetic field created by said magnet.
 5. The toolaccording to claim 4 wherein said means for controlling includes meansfor doubling the frequency of said synchronization signal.
 6. The toolaccording to claim 1, further including a downhole sensor generating asignal and means for modulating the magnetic field created by saidmagnet in response to said signal from said downhole sensor.
 7. The toolaccording to claim 1, further including a downhole sensor generating asensor signal, said shield being moveable in response to said sensorsignal so as to modulate the magnetic field created by said magnet.
 8. Alow frequency magnetic signaling device, comprising: a permanent magnet;and a magnetically permeable shield moveable relative to said magnet.